Several processes for extraction of metals from their oxides have used molten salt electrolysis on an industrial scale since the invention of the Hall-Héroult cell for aluminum production in 1886 (U.S. Pat. No. 400,664; herein incorporated by reference in its entirety). When the raw material is not water-soluble and the product metal is very reactive, as with aluminum, it is most advantageous to dissolve the raw material in a molten salt electrolyte and perform electrolysis in a high temperature cell.
While the Hall-Héroult achieved a breakthrough in aluminum production, researchers and inventors since then have been trying for decades to improve the anode to produce oxygen instead of CO2 as the anode product. A recent invention called Solid Oxide Membrane (SOM) Electrolysis accomplishes this by adding a solid electrolyte between the molten salt and anode (see, for example, U.S. Pat. Nos. 5,976,345 and 6,299,742; each herein incorporated by reference in its entirety). The process, shown schematically in FIG. 1 for metal production, consists of a metal cathode, a molten salt electrolyte bath which dissolves the metal oxide which is in contact with the cathode, a solid oxygen ion-conducting membrane (SOM) typically consisting of zirconia stabilized by yttria (YSZ) or other oxide-stabilized zirconia (e.g. magnesia- or calcia-stabilized zirconia, MSZ or CSZ) in contact with the molten salt bath, an anode in contact with the solid oxygen ion-conducting membrane, and a means of establishing a potential between the cathode and anode. The metal cations are reduced to metal at the cathode, and oxygen ions migrate through the SOM to the anode, where they are oxidized to produce oxygen gas.
The SOM process has made significant progress toward the production of other metals such as magnesium, tantalum and titanium (See, e.g., U.S. Pat. No. 6,299,742; Britten et al., Metall. Trans. 31B:733 (2000); Krishnan et al., Metall. Mater. Trans. 36B:463-473 (2005); Krishnan et al., Scand. J. Metall., 34(5):293-301 (2005); and Suput et al., Mineral Processing and Extractive Metallurgy 117(2):118-122 (2008); each herein incorporated by reference in its entirety). This process runs at high temperature, typically 1000-1300° C., in order to maintain high ionic conductivity of the SOM. The most promising anode materials for the process are an oxygen-stable liquid metal, such as silver or its alloys with copper or tin (International Patent Application No. PCT/US2006/027255; herein incorporated by reference in its entirety). This leads to the use of a device which can establish a good electrical connection between that anode and the DC current source, known as the anode current collector. The current collector, like the anode itself, must be stable in liquid metal or make good contact with oxygen stable electronic oxides or cermets, and must conduct electricity well from ambient temperature to the high process temperature.
To date, only iridium is known to satisfy these criteria for the current collector in a liquid metal anode. Solid oxide fuel cells (SOFC) use scale-forming oxides, but the higher temperature of SOM electrolysis than SOFC makes it relatively difficult to use the SOFC current collector approaches. Most oxidation-resistant steels and nickel alloys rapidly oxidize at the very high temperature of SOM Electrolysis, and some refractory metals such as platinum dissolve in liquid silver. Oxidation-resistant alloys also generally have significantly lower electrical conductivity than purer metals.
Thus, there remains a need for more efficient and scalable apparatuses and processes to produce oxygen instead of carbon dioxide as the anode product during production of metals from the corresponding metal oxides. There also remains a need for stable and inexpensive anode systems to process metal oxides into pure metals. In particular, there remains a need for apparatuses and methods that conduct current at high temperature in an oxygen generating environment. This invention addresses these needs.